Translucent rare earth oxide sintered article and method for production thereof

ABSTRACT

A high-purity rare earth metal oxide material powder of purity of 99.9% or over, of which Al content is 5-100 wtppm in metal weight and Si content is 10 wtppm or under in metal weight, and a binder are used to prepare a molding body of which molding density is 58% or over of the theoretical density. The binder is eliminated by thermal treatment, and then the molding body is sintered in an atmosphere of hydrogen or a rare gas or a mixture of them or in a vacuum at a temperature being not lower than 1450° C. and not higher than 1700° C. for 0.5 hour or over to prepare A transparent sintered rare earth metal oxide body represented by a general formula R 2 O 3  (R being at least one element of a group comprising Y, Dy, Ho, Er, Tm, Yb and Lu).

FIELD OF THE INVENTION

[0001] The present invention relates to a transparent sintered rareearth metal oxide body represented by R₂O₃ (R being at least one elementof a group comprising Y, Dy, Ho, Er, Tm, Yb and Lu) and a productionmethod thereof. The sintered body of this invention can be usedsatisfactorily, for example, as materials for infrared rays transmissionwindows, polarization plates, discharge lamp envelopes, optical parts,and laser oscillators.

PRIOR ART

[0002] The rare earth metal oxides represented by a general formula R₂O₃(R being at least one element of a group comprising Y, Dy, Ho, Er, Tm,Yb and Lu) have a cubic crystal structure and show no double refraction.Hence they can provide sintered bodies of excellent transparency whenpores and segregation of impurities are completely eliminated from them.

[0003] Among them, yttria (Y₂O₃) has a melting point of 2415° C. beingthe highest of those of rare earth metal oxides, has a good heatresistance and a good alkali resistance, and exhibits high transparencyin the infrared region. Moreover, as yttria has high thermalconductivity, it is expected as a host material for a solid-state laser.However, as yttria has a very high melting point and exhibits phasetransition (between cubic crystal and hexagonal crystal) in theneighborhood of 2280° C., it is difficult for the existing singlecrystal synthetic techniques to synthesize large crystals of excellentoptical properties. On the other hand, as its ceramics (polycrystals)can be synthesized at relatively low temperatures below its meltingpoint, efforts have been made extensively to apply its ceramics as hightemperature window materials for infrared rays, discharge lampenvelopes, corrosion resistant members, etc.

[0004] In preparing transparent sintered bodies, not limited in those ofrare earth metal oxides, what is most important is whether eliminationof pores can be well done during grain growth in the sintering stage. Atechnique of adding a sintering additive is normally used to control thevelocity of grain growth. In the greater part of the production methodsof yttria which have been reported up to the present, a sinteringadditive is added.

[0005] The following methods are known as production methods oftransparent yttria using a sintering additive:

[0006] (1) A method of adding ThO₂ and sintering in a hydrogenatmosphere at 2100° C. or over (Ceramic Bulletin Vol. 52, No5(1973));

[0007] (2) A method of sintering Y₂O₃ powder, to which AlF₃ is added, bya vacuum hot press (Japanese Provisional Patent Sho 53-120707);

[0008] (3) A similar method of hot pressing Y₂O₃ powder to which LiF orKF is added (Japanese Provisional Patent Hei 4-59658); and

[0009] (4) A method of adding La₂O₃ or Al₂O₃ and sintering in a low O₂atmosphere (Japanese Provisional Patent Sho 54-17911, JapaneseProvisional Patent Sho 54-17910).

[0010] In the method of (1), radioactive thoria is added as a sinteringadditive, which is not easy to obtain and handle, although its additionwill provide a sintered body of a relatively high transparency.Moreover, as sintering is carried out at a high temperature for a longperiod of time, the mean grain size is as large as 100 μm or over, andthe strength of the material is extremely low. Hence the sintered bodyis not applicable to daily use. The hot pressing method of (2) allowssintering at a relatively low temperature. It, however, can only providesintered bodies of which in-line spectral transmittance in the visibleregion is about 60%.

[0011] According to the method of (3), sintered bodies of which in-linespectral transmittance in the infrared region at a wavelength of 2 μm orover is about 80% can be produced by hot press at a temperature of 1500°C. or over. The transmittance in the visible region is not certainbecause it is not indicated therein. However, the fluorides which areadded as sintering additives have low melting points (LiF: 842° C.; KF:860° C.) and may evaporate in the sintering process to generate a gap inthe velocity of grain growth between the circumferential portion and theinternal portion of the sample. Therefore, it is estimated to bedifficult to produce a homogeneous sintered body when the sample isthick. According to Majima, et al. (Journal of Japan Inst. Metals Vol.57, No. 10 (1993) P.1221-1226), it is reported that when LiF is used asan additive and hot pressing is used, even if the amount of additive isoptimized, fluorine will remain in the central portion of the sample,and the transmittance thereof will be lower in comparison with that ofthe peripheral portion of the sample. Accordingly, it is not easy to usefluorides as sintering additives to produce large-sized and thicksintered bodies.

[0012] According to the method of (4), La₂O₃ is added by about 6 to 14mol %, and La₂O₃ which can not be solid dissolved tends to form asegregation phase (refer, for example, to Journal of Materials Science24 (1989) 863-872), hence it is not easy to prepare an opticallyhomogeneous sintered body. According to the method of Al₂O₃ addition,the amount of the additive is from 0.05 wt % to 5 wt %, and high densitybodies are prepared by liquid phase sintering at a temperature not lowerthan the eutectic temperature between Y₄Al₃O₉ and Y₂O₃ (1920° C.).However, in spite of the sintering at a high temperature, thetransmittance of the sintered bodies thus obtained is only 80%, at thehighest, of the theoretical transmittance.

[0013] On the other hand, production methods of yttria with no sinteringadditive are disclosed in Japanese Patent No. 2773193 and JapaneseProvisional Patent Hei 6-211573. According to Japanese Patent No.2773193, yttria powder having BET value of 10 m²/g or over is hotpressed to achieve maximum density of 95% or over of the theoreticaldensity, and after that, HIP treatment is given. The transmittance ofthe sintered body thus obtained is as good as about 80% in the infraredregion of 3-6 μm of wavelengths, but the transmittance in the region of0.4-3 μm in wavelength remains to be about 75% in average. Thisinsufficient transparency in the shorter wave region in spite of the HIPtreatment may be attributed to the use of ultra-fine powder, which isdifficult to handle, as the starting material; although the surface ofthe sintered body is densified by hot pressing, large voids which aredifficult to be removed even by the HIP treatment tend to remain in theinner part of the sample.

[0014] According to the method of Japanese Provisional Patent Hei6-211573, transparent bodies are prepared by CIP-molding easilysinterable powder having a mean particle size of 0.01-1 μm and vacuumsintering at 1800° C. or over or giving HIP treatment at 1600° C. orover. It is stated that the sintered bodies obtained by this method havea mean in-line spectral transmittance as high as 80% or over in thevisible region, and that it is possible to prepare a sintered body whichcan make laser oscillation by adding a luminiferous element. However, toprepare a sample of high transparency, it is necessary to executesintering at a high temperature around 2000° C. in either case of vacuumsintering or HIP treatment. In the case of industrial continuousproduction, the degradation of the sintering furnace is excessive andthe maintenance of the furnace is troublesome. Moreover, when thewavelength gets shorter, the transmittance will drop markedly (when thewavelength is reduced from 1000 nm to 400 nm, the transmittancedecreases by 10 or more percent). Hence this method is not appropriatefor producing optical parts of which transparency in the visible regionis important.

[0015] Generally, mother salts of rare earth metal oxide materialpowders used in the conventional methods are oxalates. The materialpowders which are obtained by calcining oxalates are composed of highlyaggregated secondary particles and their particle size distributions areinhomogeneous. Hence packing by molding can not be done sufficiently,and it is not easy to prepare high density bodies. To improve thispoint, methods of preparing transparent bodies, which use easilysinterable material powders and low temperature sintering, have beendisclosed in recent years (for example, Japanese Provisional Patent Hei9-315865, 10-273364, 11-189413 and 11-278933).

[0016] According to these methods, carbonates are used as the mothersalts, and carbonates are calcined to obtain powders of which particlesize distributions are relatively even and which show less aggregation.These powders are used as the starting materials to obtain sinteredbodies. However, the in-line spectral transmittance of the sinteredbodies obtained by these methods in the visible region is about 70% atthe highest, and when this figure is compared with the theoreticaltransmittance (≈82%), it is hard to say that they are transparent bodiescomparable to single crystals.

[0017] So far, the existing production methods of transparent yttriahave been described. It should be noted that there is no method toeasily and industrially produce sintered bodies which have excellenttransparency comparable to that of single crystals in from the visibleregion to the infrared region. There are hardly any reports on theproduction of transparent sintered rare earth metal oxide bodies using arare earth element other than yttria because rare earth elements arerelatively expensive and there are no specific applications for them,although the production conditions are almost comparable to those of thecase of yttria.

SUMMARY OF THE INVENTION

[0018] An object of the present invention is to provide a sintered rareearth metal oxide body which exhibits good transmittance in from thevisible region to the infrared region by using an industrially feasibletechnique, and a production method thereof.

[0019] The transparent sintered rare earth metal oxide body according tothe present invention is represented by a general formula R₂O₃ (R beingat least one element of a group comprising Y, Dy, Ho, Er, Tm, Yb andLu), the in-line spectral transmittance of the sintered body of 1 mmthick is 80% or over for wavelengths ranging from 500 nm to 6 μm beyonda specific absorption wavelength, and the content of Al in the sinteredbody, as an amount of Al metal, is 5 wtppm or over and 100 wtppm orunder. Al of 5 wtppm or over is needed to density the sintered body, andin particular, to completely eliminate pores so as to obtain an in-linespectral transmittance of 80% or over. Al exceeding 100 wtppm causes Alto segregate in grain boundaries and produce foreign phases, resultingin a decrease in the in-line spectral transmittance.

[0020] When the mean grain size of the sintered body is large, even ifthe Al content is the same, foreign phases tend to deposit in the grainboundaries. Hence the mean grain size of sintered body is preferably 2μm or over and 20 μm or under.

[0021] As Si increases the mean grain size of the sintered bodies, it isdesirable to keep the Si content in the sintered bodies at 10 wtppm orunder in metal weight so as to set the mean grain size at 2-20 μm.

[0022] In the present invention, the transparency of the sintered bodyis improved by Al addition of 5-100 wtppm whereas the prior art improvesthe transparency of the sintered bodies by the use of CaO or MgO. Henceit is desirable to keep the CaO content or the MgO content below 5wtppm. When CaO or MgO is solid dissolved in Y₂O₃, the sintered bodywill tend to get colored. This is attributed to that the difference inthe electric charge between trivalent Y ion and divalent Ca ion ordivalent Mg ion tends to generate defects which cause color absorption.

[0023] In the production method of transparent sintered rare earth metaloxide body according to the present invention, the Al content in metalweight is 5-100 wtppm, the Si content in metal weight is 10 wtppm orunder, and high-purity rare earth metal oxide material powder of 99.9%or over in purity is used to prepare moldings of which molding densityis 58% or over of the theoretical density. The binder is removed fromthe moldings by thermal treatment, then the moldings are sintered in anatmosphere of hydrogen or a rare gas or a mixture of them or in a vacuumat a temperature being not lower than 1450° C. and not higher than 1700°C. for 0.5 hour or over. This method can produce sintered bodies ofwhich in-line spectral transmittance, when measured on a sintered bodyof 1 mm thick, is 80% or over for wavelengths ranging from 500 nm to 6μm beyond the specific absorption wavelength.

[0024] The mean grain size of the sintered body is preferably 2-20 μm,the sintered body is preferably substantially free of deposition of anyforeign phases containing Al in grain boundaries, and the materialpowder and the molding process are preferably controlled so as to keepCaO or MgO below 5 wtppm each in the sintered body.

[0025] In the following, the Al content and Si content are indicated inmetal weights.

[0026] To solve the above-mentioned problems, the present inventorsinvestigated the problems in many aspects and found that a sintered rareearth metal oxide body can be produced, of which in-line spectraltransmittance, when measured on a sintered body of 1 mm thick, is 80% orover for wavelengths ranging from 500 nm to 6 μm beyond the specificabsorption wavelength. To this end, the material purity, the Al contentand the molding density are controlled to prepare moldings, then thebinder is removed from the moldings by thermal treatment, and themoldings are sintered in an atmosphere of hydrogen or a rare gas or amixture of them or in a vacuum at a temperature being not lower than1450° C. and not higher than 1700° C. for 0.5 hour or over.

[0027] In the sintering of the rare earth metal oxide according to thepresent invention, an extremely small amount of Al (5 wtppm-100 wtppm inmetal weight) plays a very important role as a sintering additive. Itshould be noted that in the present specification the contents of Al andSi are represented in weight ratio in metal weight if not specifiedotherwise. The density of the molding is represented by a ratio to thetheoretical density.

[0028] As mentioned in the section of prior art, a variety of techniquesof adding a sintering additive have been disclosed, but in most of thesecases the sintering additive segregates in the grain boundaries toreduce the velocity of grain boundary migration, which in turn controlsthe velocity of grain growth and achieves maximum densification. Thedetails of the maximum densification mechanism through sintering when anextremely small amount of Al is contained according to the presentinvention is not certain. However, Al exhibits the effects as a maximumdensification promoter only when the mean grain size of the sinteredbody is in a range of about 2 μm-20 μm, and when the mean grain size islarger than that, foreign phases containing Al are generated.

[0029] When the sintering temperature is below 1450° C., irrespective ofpresence of Al, the maximum densification through grain growth will notproceed sufficiently. Hence only opaque or semi-transparent sinteredbodies can be obtained. In this case, the mean grain size is normallyless than 2 μm. When the sintering temperature is not lower than 1450°C. and not higher than 1700° C., the Al content is 5-100 wtppm and themolding density is 58% or over of the theoretical density, the meangrain size of the sintered bodies produced is within a range of 2-20 μm,depending on the sinterability of the material used, and sintered bodiesof excellent transparency can be obtained. When a sample of which Alcontent is less than 5 wtppm is sintered in a similar manner, the meangrain size is also about 2-20 μm but the sintered bodies obtained issemi-transparent or opaque. On the other hand, in the case of a sampleof which Al content exceeds 100 wtppm, the sintered bodies exhibitgreater grain growth in comparison with the sample of which Al contentis 100 wtppm or under, and the mean grain size is larger. However, thesintered bodies obtained are semi-transparent or opaque just like thoseof which Al content is less than 5 wtppm. Al as a sintering additiveworks as a maximum densification promoter when its content is within arange of 5-100 wtppm, and satisfactory transparent bodies can beobtained only in that case. However, when Al exceeds 100 wtppm, itmainly works as a grain growth promoter, and satisfactory transparentbodies can not be obtained because pores can not be eliminatedsufficiently.

[0030] On the other hand, when sintering is made at a temperatureexceeding 1700° C., grain growth proceeds significantly irrespective ofthe presence of Al. As pores are not eliminated sufficiently, it is hardto produce sintered bodies of sufficient transparency. In this case, themean grain size is, for example, 25 μm or over. At sinteringtemperatures over 1700° C., even if the Al content is as extremely smallas 5-100 wtppm, segregation phases of Al are generated in grainboundaries. Deposition of Al depends on the mean grain size of thesintered bodies, and when the mean grain size is 20 μm or under, nodeposition of Al is observed in any sintering atmospheres. However, whenthe mean grain size of sintered bodies exceeds 20 μm, segregation of Alstarts to occur in grain boundaries, and when the mean grain size is 30μm or over, this phenomenon becomes conspicuous.

[0031] Accordingly, Al exhibits effects as a maximum densificationpromoter when its content is only in the range of 5-100 wtppm, andsintered bodies of excellent transparency can be produced only whensintering is made in a temperature range of 1450° C. or over and 1700°C. or under, which produces no Al deposition, in such a way that themean grain size is not smaller than 2 μm and is not larger than 20 μm.It should be noted that to ensure that an extremely small amount of Alfully exhibits its maximum densification promotion effects to producesintered bodies of excellent transparency, it is necessary to strictlycontrol the Si content in the raw material. The Si content must be keptnot higher than 10 wt ppm and the molding density must be 58% or over ofthe theoretical density.

[0032] In a high-purity rare earth metal oxide powder of 99.9% or overavailable in the market as a rare earth element, a content of eachelement including as an impurity is about a few wtppm and about 10 wtppmat the highest. For example, the content of CaO or MgO is 5 wtppm orunder. However, Si is contained by about 10 wtppm in many cases, and Siof as high as several tens of wtppm or over is contained in some cases.This is attributed to that the crucible used for sintering a rare earthmaterial is normally made of quartz and adhering water slightly reactswith the quartz crucible to make Si contaminate with the materialpowder. Moreover, the reactor vessel may be made of glass or lined withglass, or Si may be contained in a precipitating agent in some cases.The concentration of Al as an impurity in high-purity rare earthmaterials is less than 5 wtppm. Unintended contamination with Al in theproduction process of sintered bodies can be prevented by using plasticballs such as nylon balls rather than alumina balls for crushing thematerial powder, and using a high-purity alumina crucible forcalcination to reduce the reactivity of the crucible. When thesemeasures are taken and Al is not added intentionally, the Alconcentration in the sintered bodies can be kept below 5 wtppm.

[0033] As Si generates liquid phases in grain boundaries and promotesgrain growth, when the content of Si is high, it will cancel the maximumdensification promotion effect of the extremely small amount of Al.Hence Si present in the rare earth metal oxide material powder to beused should be kept not higher than 10 wtppm, and preferably not higherthan 5 wtppm. As the greater part of Si present in the material comesfrom the calcining crucible, it is possible to secure a material havinga smaller Si content by using, for example, an alumina crucible forcalcination. Si may come from deionized water or distilled water. Henceuse of ultra-pure water is preferable. As for the alumina crucible, itis preferable to use a high-purity alumina crucible of, for example, 99%alumina so as to prevent contamination with Al from the crucible.

[0034] According to the present invention, it is necessary to preparehomogeneous and high-density moldings having no large pores nor voidsinside. Common transparent ceramics are sintered at temperatures lowerthan their melting points by about 100° C.-300° C., and their mean grainsizes are about 50 μm or over. This means that to eliminate pores insidethe moldings by grain growth, when moldings with many pores (having alower molding density) are to be sintered, grains are made to growsignificantly to produce high density bodies. On the other hand, thesintered bodies according to the present invention are sintered atrelatively low temperatures of 1700° C. or under, which do not producedeposition of Al, and their mean grain size is as relatively small as 20μm or under. Accordingly, to prepare sintered bodies with excellenttransparency without relying on the excessive grain growth for poreelimination, it is necessary to prepare and sinter homogeneous andhigh-density molding bodies.

[0035] Inside molding bodies of which molding density is less than 58%there are a large number of pores due to insufficient packing. It is noteasy to achieve proper maximum densification of these molding bodies ata low temperature not higher than 1700° C. On the other hand, insidemolding bodies of which molding density is 58% or over there are arelatively smaller number of pores, and these molding bodies can bedensified sufficiently at a low temperature. Accordingly, to preparesintered bodies of excellent transparency, of which in-line spectraltransmittance, when measured on a sintered body of 1 mm thick, is 80% orover in the region ranging from 500 nm to 6 μm in wavelength beyond thespecific absorption wavelength, it is necessary to set their moldingdensity at 58% or over, and preferably at 60% or over.

[0036] Embodiments

[0037] In the following, sintered bodies of embodiments and theproduction methods thereof will be described.

[0038] To prepare sintered bodies, a high-purity easily sinterablematerial powder of purity of 99.9% or over, of which Si content is 10wtppm or under, is used. Generally, an element of rare earth materialsis prepared by separation and refining by solvent extraction from orescontaining a plurality of rare earth elements and calcination ofprecipitates of oxalates. Accordingly, material powders which have notbeen subjected to full separation and refining may contain some rareearth elements other than the main component. In some cases, a rareearth element contained as an impurity may exhibit its specificabsorption and there is a fear of coloring of the sintered bodies, andthis is not desirable. Transition elements such as Fe are not desirablebecause they work as a coloring source similarly. Hence it is necessaryto select a fully refined starting material. However, in the case oflaser oscillator materials, a laser active element such as Nd or Yb isadded, and in the case of colored glasses, a coloring element is added.

[0039] The sinterability of a material powder depends on its mothersalt. For example, in the case of yttrium, the sinterability is normallyin the following descending order: (1) carbonate, (2) hydroxide, (3)oxalate, (4) ammonium sulfate, (5) sulfate (based on, for example, L. R.Furlong, L. P. Domingues, Bull. Am. Ceram. Soc, 45, 1051 (1966)).However, the kinds of these mother salts are not particularly limited.Any mother salt which is easy to obtain may be used.

[0040] The primary particle size of the material powder to be used isnot specified particularly. Any powder which is suited to the moldingand sintering processes may be selected. Ultra-fine powder has a highsintering activity and can be densified well at a relatively lowtemperature, but its handling is not easy. Moreover, ultra-fine powderhas many aggregated particles and it is not easy to increase the moldingdensity thereof. In the case of coarse powder, packing is easy but thesintering activity is low, and it can not be densified at a lowtemperature. Accordingly, from the viewpoints of ease in sintering,packing and handling, the specific surface area of the material to beused is preferably about 3-12 m²/g and more preferably about 4-10 m²/g.Moreover, it is most preferable to use a material powder having ahomogeneous particle size distribution and showing little aggregation.

[0041] Next, moldings of a desired configuration are formed by using therare earth metal oxide material powder. Molding methods for ceramicsinclude extrusion, injection molding, pressing and casting. In theembodiments molding is not limited to any specific technique; anytechnique, which can achieve the molding density of 58% or over andcauses little contamination with impurities, may be used. At this time,if necessary, Al being a sintering additive is added so that it isdispersed homogeneously depending on the molding method used. Forexample, in the case of pressing, an appropriate amount of Al is addedto the slurry for granule preparation. The slurry is fully mixed in, forexample, a ball mill, then dried by a spray drier, etc. and formed intogranules for molding.

[0042] As for the timing of Al addition, it is not particularlyspecified provided Al can be homogeneously dispersed in the entiremoldings. For example, it may be added in the material compounding stageor the calcination stage without any problems. To make an extremelysmall amount of Al exhibit its effects fully, it is most preferable tomix Al in the material.

[0043] Its addition form is not particularly specified. For example, ifAl is to be mixed in the molding stage, an appropriate amount of analuminium compound such as alumina sol, Al₂O₃ powder or R₃Al₅O₁₂ powder(R being any of Y, Dy, Ho, Er, Tm, Yb and Lu). If Al is to be added inthe material compounding stage, it may be added in the form of aluminiumchloride or aluminium hydroxide. As for the purity of an additive, it isnot particularly specified because the amount of addition is very small.However, like the material powders, it is preferable to use ahigh-purity additive. If an additive is to be added in the form ofpowder, it is preferable to use a powder of which particle size iscomparable to the primary particle size of the material powder orsmaller than that.

[0044] Molding bodies are subjected to heat treatment to remove thebinder. The treatment temperature, duration and atmosphere varydepending on the kind of the molding additive added. If pores on thesurface of the sample are closed off, it will become difficult to removethe binder. Hence the binding removal is done by taking much time at atemperature below the temperature at which the pores on the surface ofthe sample are closed. The latter temperature depends on the calciningtemperature, sinterability of the material powder to be used and packingof the molding bodies and is normally about 900° C.-1400° C. Hence it ispreferable to remove the binder at a temperature below this temperature.As for the atmosphere, the oxygen atmosphere is the most common one, butthe binder removal may be done, if necessary, in an atmosphere of wethydrogen or in an atmosphere of Ar or under reduced pressure.

[0045] After the completion of binder removal treatment, the sample issintered in an atmosphere of hydrogen, a rare gas or a mixture of themor in a vacuum at a pressure not being lower than 1450° C. and not beinghigher than 1700° C. for 0.5 hour or over. Moreover, after thecompletion of binder removal, it is effective to close the pores of thesample by primary sintering and then subject the sample to HIPsintering. As for the sintering time, 0.5 hour or over is needed tohomogeneously sinter the entire molding bodies. The sintering time isnot particularly specified provided it is longer than that. It isnormally sufficient to sinter for about 2 to 10 hours when the samplethickness is about 1 to 5 mm. In the case of pressure sintering, it issufficient to sinter for about 0.5 to 2 hours.

[0046] In the following, some embodiments will be described but thepresent invention is not limited in any way by any of these embodiments.

EMBODIMENT 1

[0047] According to the technique of Japanese Provisional Patent Hei11-157933, Y₂O₃ material powder of which mean primary particle size was0.3 μm, purity was 99.9% or over, and Si content was 3 wtppm wasprepared. To be more specific, an aqueous solution of a nitrate ofyttrium, an aqueous solution of urea and an aqueous solution of ammoniumsulfate were mixed to obtain yttrium:urea:ammonium sulfate=1:6:1 inmolar ratio. The mixture was made to react hydrothermally in anautoclave at 125° C. for 2 hours to obtain a carbonate of yttrium. Thecarbonate obtained was washed with pure water and dried. Next this drypowder was calcined in an alumina crucible in the atmosphere at 1200° C.for 3 hours to obtain a material powder.

[0048] 60 g of a plasticizer being Ceramisol C-08 (NOF Corp., Ceramisolis a trade name) and 300 g of methyl cellulose as a binder were added to2 kg of this material powder. Alumina sol (NISSAN CHEMICAL INDUSTRIES,LTD.) equivalent to 50 wtppm as an amount of Al metal was added to thematerial powder as a sintering additive. 4 kg of pure water was added tothe material powder, and the mixture was mixed in a ball mill using anylon pot and nylon balls for 100 hours. The resulted slurry was heatedto concentrate it to a viscosity allowing extrusion. The material waspassed through a triple roll mill five times to improve its homogeneity.The material thus obtained was molded by an extruder into a bodymeasuring 60 mm×200 mm×3 mm.

[0049] This molding was dried sufficiently, then its temperature wasraised to 600° C. at 20° C./hr and the molding was held at 600° C. for20 hours to defat it. The density of this molding body was determined tobe 59.8% by Archimedes' method. To fully defat it, the molding body wasfurther raised up to 1200° C. and held at that temperature for 10 hours.After that, the molding body was sintered in a vacuum furnace at 1650°C. for 8 hours. At this time, the temperature rise rate was 300° C./hrup to 1200° C., and after that the rate was 50° C./hr, and the degree ofvacuum in the furnace was 10⁻¹ Pa or under.

[0050] The sintered body thus obtained was mirror-polished with diamondslurry, and the in-line spectral transmittance was measured with aspectrophotometer. As a result, the in-line spectral transmittances atthe wavelengths of 500 nm and 800 nm were 80.6% and 82.1% (the samplethickness: 1 mm), respectively. The transmittances in the infraredregion were 83.2% and 84.1% at the wavelengths of 3 μm and 6 μm,respectively.

[0051] This sample was subjected to thermal etching in the atmosphere at1500° C. for 2 hours, and its microstructure was observed under anoptical microscope. As a result, the mean grain sized was found to be12.6 μm. The mean grain size was determined with a SEM or the like byfreely drawing a line on a high resolution image of the sample. When thelength of the line was C, the number of grains on this line was N, andthe magnification was M, the mean grain size was given by an equationthe mean grain size=1.56C/(MN). Moreover, when the density of thesintered body was determined by Archimedes' method, it was found to be99.97% of the theoretical density. This sintered body was dissolved bymeans of an autoclave, and the amounts of Al and Si were determined byICP method. Al was 47 wtppm and Si was 3 wtppm.

EMBODIMENTS 2-7

[0052] Various sintered rare earth metal oxide bodies were prepared in away similar to that of embodiment 1. In every sample, the purity of thematerial used as a rare earth element was 99.9% or over, Si was 10 wtppmor under, and the molding density was 58% or over. The sinteringconditions, the Al content, the in-line spectral transmittance of 1 mmthick sample, and the mean grain size are shown in Table 1. Themeasurement wavelength used for the in-line spectral transmittance was500 nm for Yb₂O₃ and Lu₂O₃. Wavelengths being free from any influencesof specific absorption were selected for other sintered bodies. TABLE 1Embodiment 2-Embodiment 7 Mean In-line spectral Sintering graintransmittance/ temp./° C. × Al/ size/ % (measure time/h wt ppm μmwavelength/nm) Embodiment 2: Dy₂O₃ 1675 × 8 90 16.9 81.2 (600)Embodiment 3: Ho₂O₃ 1625 × 5 64 7.7 80.3 (580) Embodiment 4: Er₂O₃  1625× 10 31 9.0 80.5 (600) Embodiment 5: Tm₂O₃ 1650 × 7 25 13.2 81.1 (575)Embodiment 6: Yb₂O₃ 1650 × 7 10 10.8 80.9 (500) Embodiment 7: Lu₂O₃ 1680 × 10 52 19.3 81.5 (500)

[0053] The in-line spectral transmittances of the sintered bodiesprepared in embodiments 1 through 7 were measured in wave lengthsranging from 1 μm to 6 μm (beyond the specific absorption wavelength),and each of them was 82% or over in all cases. These results show thatsintered bodies having excellent transparency in from the visible regionto the infrared region can be prepared by these embodiments.

COMPARATIVE EXAMPLES 1-5

[0054] According to the technique of Japanese Provisional Patent Hei11-157933, Y₂O₃ material powder was prepared. In calcining the materialpowder, a quartz crucible was used, and material powders havingdifferent Si contents were obtained by changing the sampling points inthe crucible. In calcining the materials used for comparative example 1and comparative example 5, high-purity alumina crucibles were used. Thematerial powders thus obtained were used to prepare sintered yttriabodies having different Al contents in a manner similar to embodiment 1.The amount of Si contained in the material, the amount of Al containedin the sintered body, and the in-line spectral transmittance at thewavelength of 500 nm (sample thickness: 1 mm) are shown in Table 2. Themolding densities were 58% or over in all of the cases. TABLE 2Comparative Example 1-Comparative Example 5 In-line spectral Si/wt ppmAl/wt ppm transmittance/% Comparative example 1 3 2 63 Comparativeexample 2 21 15 48 Comparative example 3 12 30 51 Comparative example 430 50 45 Comparative example 5 3 115 57

[0055] As is the case of comparative example 1, when the amount of Alcontained in the sintered body is little, its effect is not exhibitedfully. Hence the transparency is not high although the mean grain sizeis 11 μm which is substantially comparable with that of embodiment 1. Asis the case of comparative example 5, when the Al content exceeds 100wtppm, the mean grain size is 30 μm which is twice or more of that ofembodiment 1, and as sufficient maximum densification is not done, thetransparency is not high. This sample was observed under a SEM with EDX(energy dispersion type X-ray analysis). Segregation phases of Al werefound in grain boundaries. Conversely, comparative examples 2 through 4indicate that even when the Al content in the sintered body is withinthe range of 5-100 wtppm, if the amount of Si contained in the materialexceeds 10 wtppm, a sufficient transparency can not be obtained.Accordingly, in the light of these comparative examples it was foundthat to prepare a sintered body of excellent transparency, it isnecessary to strictly control the amount of Si contained in the materialand the amount of Al contained in the sintered body.

EMBODIMENTS 8, 9 AND 10 AND COMPARATIVE EXAMPLES 6, 7 AND 8

[0056] Er₂O₃ material powder of which purity was 99.9% or over, Sicontent was 3 wtppm and a primary particle size was 0.35 μm was moldedby CIP. By changing the molding pressure, molded bodies having differentmolding densities were produced. Then sintered bodies were prepared in amanner similar to that of embodiment 4. The molding density and thesintered body's in-line spectral transmittance (t=1.0 mm) at thewavelength of 600 nm are shown in Table 3. The amount of Al contained inthe sintered body was within the range of 55-60 wtppm in all the cases.TABLE 3 Molding Density and Transmittance Molding Mean grain density/%Transmittance/% size/μm Comparative example 6 49.3 — 25.2 Comparativeexample 7 531 45.3 17.3 Comparative example 8 57.6 67.5 18.6 Embodiment8 58.2 80.2 13.2 Embodiment 9 60.5 80.6 12.7 Embodiment 10 64.4 81.110.8

[0057] In comparative example 6, the grain growth was marked incomparison with other cases, a large number of pores remained inside thesintered body, and segregation of Al was also observed. As it was anopaque body, it was impossible to measure the transmittance. Comparativeexamples 7 and 8 and embodiments 8, 9, and 10 show that thetransmittance increases with the increase in molding density, and thatthe molding density of 58% or over is needed to obtain a sintered bodyof excellent transparency of 80% or over.

EMBODIMENTS 11-14 AND COMPARATIVE EXAMPLES 9-12

[0058] Alumina sol was added to Yb₂O₃ material powder of which materialpurity was 99.9% or over and Si content was 2 wtppm to set the Alcontent in the sintered body at 50 wtppm, and molding bodies having amolding density of 59.5% were prepared in a manner similar to that ofembodiment 1. The molding bodies were sintered at various sinteringtemperatures for 10 hours to prepare sintered Yb₂O₃ bodies. Thesintering temperature, the mean grain size of the sintered bodyobtained, and the in-line spectral transmittance thereof at thewavelength of 500 nm are shown in Table 4. When the sinteringtemperature was within the range of 1450-1700° C., the mean grain sizeswere 2-20 μm, and the in-line spectral transmittance was 80% or over.When the temperature was outside this range, the in-line spectraltransmittance decreased extremely. TABLE 4 Sintering Temperature andIn-line Spectral Transmittance Sintering Mean grain In-line spectraltemp./° C. size/μm transmittance/% Comparative example 9 1400 1.2 —Comparative example 10 1430 1.7 46.8 Embodiment 11 1460 2.0 80.0Embodiment 12 1600 7.8 80.3 Embodiment 13 1650 11.0 80.9 Embodiment 141695 18.4 81.1 Comparative example 11 1720 28.2 54.3 Comparative example12 1750 41.6 30.8

EMBODIMENT 15

[0059] Easily sinterable yttria material powder was prepared in a mannersimilar to that of embodiment 2 of Japanese Provisional Patent Hei11-189413. To be more precise, yttrium chloride was dissolved in purewater. While the solution was kept cooled and stirred, aqueous ammoniawas added slowly to the solution dropwise to precipitate yttriumhydroxide. Next, aqueous solution of ammonium sulfate was added to thesolution and the mixture was stirred for 3 hours. The precipitate wasfiltrated and washed with pure water and dried. The precursor or yttriumhydroxide was calcined at 1100° C. to prepare a material powder. Toprevent contamination of the material with Si, the compounding of thematerial was made in a polytetrafluoroethylene container instead of aglass beaker, and an alumina crucible was used in calcining theprecursor. The purity of the material powder thus obtained wasdetermined by ICP emission analysis. The purity was 99.9% or over, andSi was 2 wtppm.

[0060] Alumina powder (TM-DAR made by DAIMEI KAGAKU; the mean primaryparticle size is 0.3 μm, and TM-DAR is a trade name) was added to thispowder. They were fully mixed and crushed in an alumina mortar. Thepowder was put into a metal mold of φ20 mm and subjected to primarymolding at 20 MPa. Then CIP molding was given under the pressure of 250MPa. The amount of Al contained in the molding and the molding densitywere measured to be 75 wtppm and 59.6%, respectively. This molding bodywas heated up to 1650° C. at a rate of 100° C./hr and kept at thattemperature for 10 hours, then it was cooled at a rate of 200° C./hr.The degree of vacuum at the time of sintering was 10⁻¹ Pa or under. Thesintered body thus obtained was evaluated in a manner similar to that ofembodiment 1. The in-line spectral transmittance at the wavelength of500 nm was 80.3%, and the mean grain size was 14.2 μm.

[0061] A sintered body to which no Al was added was also prepared in asimilar manner. Its in-line spectral transmittance was 48%, which wassubstantially comparable to that of the sintered body obtained in theembodiment of Japanese Provisional Patent Hei 11-189413 (about 45% aftersintering at 1700° C.). These findings show that with the inclusion ofan extremely small amount of Al, a sintered body of excellenttransparency can be obtained independently of the preparation method ofthe material powder used.

COMPARATIVE EXAMPLE 13

[0062] The CaO content and the MgO content in the yttrium oxide materialpowder prepared in embodiment 1 were less than 5 wtppm, respectively.CaO corresponding to 200 wtppm, in stead of alumina sol, was added tothis material powder. They were mixed by means of the nylon balls andthe nylon pot. After that, they were treated in a manner similar to thatof embodiment 1 to prepare a sintered yttria body. Both faces of thesintered body were mirror-polished with diamond slurry. When the samplethickness was 1 mm, the in-line spectral transmittance of the sinteredbody was about 80% at the wavelength of 500 mm.

[0063] The sintered yttria body of embodiment 1 and the sintered yttriabody of comparative example 13 were left to stand in a place exposed tothe sunshine for three months. The sintered body of the embodimentshowed no changes even after three months. The sintered body of thecomparative example was slightly colored yellow after one month, and itwas evidently colored yellow after three months. For the purpose ofconfirmation, a sintered yttria body having CaO content of 50 wtppm wasprepared, other conditions being similar to those of comparative example13. When it was left to stand in a place exposed to the sunshine forthree months, it was colored similarly.

1. A transparent sintered rare earth metal oxide body represented by ageneral formula R₂O₃ (R being at least one element of a group comprisingY, Dy, Ho, Er, Tm, Yb, and Lu), having an in-line spectral transmissionnot less than 80% taken at a wavelength range of 500 nm-6 μm beyond aspecific absorption wavelength on a thickness of 1 mm of the sinteredbody, and including 5-100 wtppm of Al in metal weight.
 2. A transparentsintered rare earth metal oxide body of claim 1, characterized in that amean grain size of the sintered body is 2-20 μm.
 3. A transparentsintered rare earth metal oxide body of claim 2, characterized in that aSi content of the sintered body is not more than 10 wtppm in metalweight.
 4. A transparent sintered rare earth metal oxide body of claim1, characterized in that a mean grain size of the sintered body is 2-20μm and grain boundaries in the sintered body are substantially free fromdeposited foreign phases containing Al.
 5. A transparent sintered rareearth metal oxide body of claim 3, characterized in that each content ofCaO and MgO is less than 5 wtppm.
 6. A transparent sintered rare earthmetal oxide body of claim 3, characterized in that said sintered body isa laser oscillator material containing a laser active element.
 7. Aproduction method of a transparent sintered rare earth metal oxide bodyrepresented by a general formula R203 (R being at least one element of agroup comprising Y, Dy, Ho, Er, Tm, Yb, and Lu), comprising a step forpreparing a molding body having molding density of not less than 58% ofa theoretical density from a rare earth metal oxide material powder anda binder, wherein the rare earth metal oxide material powder has apurity of not less than 99.9%, a Al content of 5-100 wtppm in metalweight, and a Si content of not more than 10 wtppm in metal weight and astep for sintering the molding body in an atmosphere of hydrogen, a raregas, or a mixture of them, or in a vacuum at a temperature not lowerthan 1450° C. and not higher than 1700° C. for not less than 0.5 hour,afier eliminating the binder from the molding body by thermal treatment.8. A production method of a transparent sintered rare earth metal oxidebody of claim 7, characterized in that the molding body is sintered tohave a 2-20 μm mean grain size.
 9. A production method of a transparentsintered rare earth metal oxide body of claim 8, characterized in thatthe molding body is sintered to be substantially free from deposition offoreign phases containing Al at grain boundaries.